土壤碳、氮循环不仅影响有机碳储存、养分转化、森林生产力，而且与全球变化存在着复杂的交互作用。土壤碳储量和碳循环在很大程度上受到氮循环的影响和限制。自1980年代以来，我国大气活性氮沉降量不断增高，引发了严重生态环境问题，已引起高度关注。已有研究显示氮沉降能提高森林净生产力和碳汇潜力，而氮沉降对森林土壤碳储量影响却存在很大争议。基于前期研究结果，亚热带林不同林龄土壤碳循环对氮沉降的响应存在显著差异，其形成机制不明确。为此，本项目拟采取长期定位观测与模拟试验相结合的试验方法，利用林龄梯度，深入探讨持续高氮沉降对亚热带林土壤微生物多样性和土壤有机碳稳定性的影响，揭示土壤微生物多样性与有机碳稳定性之间的作用关系，阐明土壤有机碳稳定性对持续高氮沉降的响应机制，为亚热带林可持续经营、提高森林增汇潜力提供理论依据。

Soil carbon and nitrogen cycles not only affect the formation and storage of soil organic matter, nutrient transformation and forest productivity, but also have a complicated linkage to global change. The storage and cycling of soil organic carbon in forest ecosystems are, to a great extent, controlled and limited by nitrogen cycling. The atmospheric reactive nitrogen deposition has been increasing in China since 1980s, which resulted in a series of serious ecological and environmental problems. Therefore, the great attention has been drawn to the effects of the increasing nitrogen deposition. Although nitrogen deposition was considered to improve forest net primary productivity and carbon sequestration potential, there was very controversial on its effects on carbon storage and carbon sink in forest soils. Based on our previous study results, there existed singnificant differences in soil carbon cycling in subtropical evergreen broad-leaved forest over age sequence. However, it is not clear what controls this differences. In the proposed research program, we are going to use an approach concentrating on the fixed observation, in which we combine long-term fixed measurement with similation experiment, using chronosequence study. This project is designed (1) to determine the effects of increasing nitrogen deposition on soil microbial diversity and soil organic carbon stability; (2) to reveal the interactions between soil microbial diversity and organic carbon stability; and (3) to elucidate the responsive mechanisms of organic carbon stability to the long-term increased nitrogen deposition in subtropical evergreen broad-leaved forest. The results of the proposed study can provide some realizable methods to enhence potential of carbon sequestration in soil and to conduct sustainable management of the subtropical evergreen broad-leaved forest. Moreover, the results can lay certain foundations for further understanding of the long-term response of forest ecosystems to global change.